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We use the penetration constants, which are independent of the excitation power density, to evaluate the penetration abilities of single-photon-excitation (SPE) at 405 nm and two-photon-excitation (TPE) at 800 nm in porcine tissues. The penetration constant of TPE is smaller than that of SPE although the transmission loss of 405 nm is much larger than 800 nm in skin tissues. On the other hand, the penetration of SPE in blood-contained systems is obviously worse than that of TPE due to the strong absorption of hemoglobin molecules at 405 nm. [ABSTRACT FROM AUTHOR]